1. Field of the Invention
This invention relates to frequency control systems of the form commonly referred to as phase locked loops, and in the preferred embodiment, to analog phase locked loops.
2. Background of the Invention
The phase locked loop ("PLL") is a widely used frequency control system commonly found in various types of electronic apparatus. The PLL can be used for a variety of applications and utilizes a voltage controlled oscillator ("VCO"). The PLL serves to slave the frequency output of the VCO to an input reference signal such that the VCO frequency tracks variations in the input reference signal. One problem common to many of the applications is the necessity for a search action to initially direct the VCO to the correct frequency required for the PLL to begin to function. This process is often referred to as achieving "lock". At the beginning of operation, absent such a search action, the VCO frequency may wander aimlessly and never achieve lock. A second problem is the requirement that this search action cease when the correct frequency is found and the PLL achieves lock. The search and lock scenario is an action that may occur many times during the time a PLL circuit is active.
The requirement for PLL search is not new. Various approaches to achieve lock have been known in the past. Other known systems for the purpose of sweeping a VCO in an analog PLL fall into two categories: 1) self starting sine wave oscillators, and 2) search circuits requiring outside start/stop commands. Each of these two types has certain disadvantages associated with it.
Self starting sine wave search circuits are usually very complicated and the search portion of the circuitry may represent an unacceptably high portion of the system. A typical prior art circuit of this type is illustrated in FIG. 1. A phase detector 4 processes an input signal 2 to provide an output error signal. This error signal is an input to a bandlimiting loop filter 13 comprised of operational amplifier ("op-amp") 8, input resistor 6, feedback resistor 10 and feedback capacitor 12. The output of the loop filter drives VCO 14 which in turn generates the PLL output signal 18. The PLL output signal 18 is also fed back via divide-by-N circuit 16 to provide a second input to phase detector 4. The circuit described to this point is a well known type 2 second order PLL. As previously mentioned, at the beginning of operation the output signal 18 may not be at the correct frequency corresponding to the value of input signal 2. Absent additional intervention, the frequency may never reach the correct value so that lock can be achieved.
The addition of feedback network 20 results in a search 5 action which directs the VCO output frequency to the correct value. Feedback network 20 may be a Wien bridge or a shunt T network. It is connected to the non-inverting input of op-amp 8 so as to provide positive feedback around the op-amp. This positive feedback network cooperates with the characteristics of the loop filter to function as a sinewave oscillator, that is to generate a low frequency sinewave to sweep the VCO. When the VCO is driven to the correct frequency and the PLL achieves lock, the feedback with the PLL in lock is enough to overcome the positive feedback of the loop filter oscillator circuit and it stops oscillating. While the circuit appears straightforward, its applications are limited. The sine wave sweep is inefficient and the circuit elements are very critical. The loop filter design goals may have to be compromised to satisfy the additional oscillator requirements.
The second category of prior art search circuits is illustrated in FIG. 2. Again, the PLL used in this illustration is of type 2, second order, and its components are the same as those described in connection with FIG. 1. In this case, the sweep action is provided by current injected into the feedback circuit of the loop filter by a switched constant current source 24. Current source 24 operates under the control of a search control signal 22 which in turn is generated by a search control circuit (not shown). The loop filter again does double duty. During search it acts as an integrator. A constant current input provided by current source 24 results in a ramp voltage provided to the VCO so as to drive it to the correct frequency and achieve lock. This type of search operation requires the additional circuitry to recognize when the PLL is not in lock and to generate the start and stop commands for the constant current source 24.